An energy-storing flywheel assembly is provided with a flywheel having a vertically-oriented axis of rotation, a shaft connected to the flywheel, and a cageless ball thrust bearing rotatably supporting a lower end of the shaft. The flywheel may be formed from a stack of flywheel discs and is preferably relatively large-radiused and massive. The ball thrust bearing includes a stator having a set of three free-floating bearing balls contained within a cylindrical stator recess; a rotor rotatably mounted over the stator which likewise contains a set of three free-floating bearing balls within a cylindrical stator recess, and a single, middle bearing ball centrally disposed between and in mutual engagement with the three bearing balls of the stator and the three bearing balls of the rotor. In operation, the relatively massive flywheel is rotated at a peripheral speed that approaches but does not exceed the speed of sound to avoid fluid drag and the danger of explosive shattering.